Characterization of Human TRPA1 and TRPV1 Channels in Response to Naturally Occurring Defensive Compounds

Characterization of human TRPA1 and TRPV1 channels in response to naturally occurring defensive compounds The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Ibarra, Yessenia Michelle. 2013. Characterization of human TRPA1 and TRPV1 channels in response to naturally occurring defensive compounds. Doctoral dissertation, Harvard University. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:11156673 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA © 2013 – Yessenia Michelle Ibarra All rights reserved. Dissertation Advisor: David E. Clapham Yessenia Michelle Ibarra Characterization of human TRPA1 and TRPV1 channels in response to naturally occurring defensive compounds Abstract The transient receptor potential channels, ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1), are non-selective cation-permeable channels that have retained their function as chemical sensors since their first appearance in metazoan species several hundred million years ago. In vertebrates, TRP channels have evolved multiple functions which make it difficult to understand exactly how they transmit signals to the brain that are interpreted very differently. For example, TRPA1 and TRPV1 are sensitive to various chemicals and activation of these channels produce sensations with opposing effects. Pain is felt when TRPV1 is activated by spider toxins, but activation by plant cannabidiol results in a pain-relieving sensation. Similarly, TRPA1 activation by delta- tetrahydrocannabinol is reported to relieve symptoms of pain, but TRPA1 activation by the active ingredient in wasabi results in a repulsive or noxious sensation. Much of what we know about TRPA1 and TRPV1 comes from the use of plant products or exposure to substances that cause or alleviate pain and inflammation. In this study, whole-cell voltage clamp recordings of heterologously expressed human TRPA1 and human TRPV1 were tested for sensitivity to a hallucinogenic plant compound, salvinorin A and an arthropod-defensive compound, para-benzoquinone. Neither compound has yet been reported to activate TRP channels but both are known to be involved in pain and inflammation signaling in humans. I show that the arthropod compound, iii para-benzoquinone, activates and desensitizes TRPA1 in a cysteine-dependent manner, but activation of TRPV1 is not dependent on cysteine reactivity. Although salvinorin A is known to be a potent agonist of the kappa-opioid and cannabinoid receptors, here I show that it also acts as a highly potent agonist of both TRPA1 and TRPV1. Its interaction with TRP channels may contribute to its antinociceptive effects in behavioral studies with animals that are reported to be independent of opioid signaling. iv Table of contents Introduction ………………………………….………………………………………….…........1 Materials and Methods……………………………………………………………………...….6 Chapter 1: An insect benzoquinone reveals a cysteine-dependent desensitization mechanism of TRPA1…………………………………………….….…....…8 Chapter 2: TRPV1 is activated by para-benzoquinone………………………….……...…49 Chapter 3: The plant hallucinogen, salvinorin A, activates TRPA1 and TRPV1……………………………………………………………….….….….…58 Conclusion……………………………………………….………………………………….....73 References……………………………………………………………………….…….…..…..77 v Introduction Chemosensation is a common trait among multi-cellular organisms. The emergence and evolution of diverse, multifunctional, ligand-activated ion channels and other receptors that activate channels through second messengers enabled all animals to detect and distinguish if chemicals are harmful. Animal behavior studies have revealed that exposure to certain chemical compounds result in an aversive reaction in some animals, but appear to be harmless to others (e.g. capsaicin laced birdseeds to deter squirrels; Fitzgerald et al., 1995). Interestingly, ion channels expressed in modern mammals still retain sensitivity to insect and plant compounds. Toxins from snakes can activate or inhibit acid sensing channels (ASICs) causing or relieving pain (Bohlen et al., 2011; Diochot et al, 2011); the mammalian voltage-gated sodium channel is blocked by the puffer fish tetrodotoxin (TTX), but the Drosophila sodium channel homologue is insensitive to TTX (Zhang et al., 2011); several plant compounds used as food flavoring are also agonists of various TRP channels while also containing antimicrobial and insecticidal properties (Park et al., 2008). A handful of mammalian sensory ion channels are known to be involved in the detection of thousands of chemicals, which make it difficult to pinpoint exactly how an ion channel can “sense” the difference between harmless and harmful. Much of pharmacological ion channel research involves the use of naturally produced plant and animal products, such as: capsaicin and resiniferatoxin (plants), tetrodotoxin and conotoxins (sea animals), double-knot toxin and myotoxin (insects). Many of these natural compounds are dangerous to humans, but other natural products that are not perceived as harmful include: menthol, cinnamon, oregano (carvacrol) and 1 rosemary (cineole). Some plant compounds have been used in naturopathic settings for centuries and are known to have antinociceptive and anti-inflammatory effects, such as cannabis (Δ-tetrahydrocannabinol) (Langford et al., 2012; Izzo et al., 2009). The natural plant compounds that we associate with having an aromatic or naturopathic property come from a family of chemicals known as terpenoids or isoprenoids. These compounds are made up of at least one isoprene unit, CH2=C(CH3)CH=CH2, but more moieties can be built upon this core to make more complicated structures. Examples of plant terpenoids are shown below in Figure 0.1. Figure 0.1: Naturally occurring plant terpenoids. A large tetraterpene, beta-carotene and smaller monoterpene, carvacrol (oregano). (Figures copied from www.internetchemie.info). Plant terpenoids are involved in the metabolism and development of plants but are also emitted into the environment for survival purposes (Langenheim, 1994). They often have a pleasant aromatic component and beneficial effects on humans, but another function of terpenoids outside of the plant structure is to act as repellants for small mammals, insects, and microbes (Gershenzon and Dudareva, 2007). For example, cinnamon and carvacrol both have antimicrobial and insecticide properties 2 and are sometimes used to treat inflammation in humans (Park et al., 2008; Tung et al.,2008; Cheng et al., 2009; Guimaraes et al., 2012; Sleha et al, 2012). Insects and marine animals also produce and emit terpenes as a defensive strategy and these terpenes are chemically similar to the plant compounds (M. Gordaliza, 2010; Conner et al., 2007). However, it is not apparent that there are any beneficial effects of insect-derived compounds on humans other than for antimicrobial purposes. Most of what is known about insect terpenes is that they function as protective substances, which indicates that these compounds are reactive and used to cause harm (Roth and Eisner 1962; Gershenzon and Dudareva, 2007). Plant and insect terpenes contain chemical groups that are unsaturated (have double bonds) and can easily form covalent bonds by sharing electrons with nucleophiles, which makes them electrophilic (see Figure 0.2; Alarie et al., 1998). Figure 0.2: Examples of reactive defensive quinones produced and emitted from animals. On the left is the large nakijiquinone, isolated from marine sponges native to Okinawa, Japan (M. Gordaliza, 2010). On the right is a small methyl 1, 4 – benzoquinone that is produced by the flour beetle (Uhruh et al., 1998). 3 Chemical detection of electrophilic compounds is a sensory modality that has been conserved for over 500 million years, from invertebrates to vertebrates and it is known that some of the insects’ detection mechanisms are mediated by TRP channels (Cosens and Manning, 1969; Matsuura et al., 2009). More specifically, in Drosophila, the TRPA channel is responsible for the detection of electrophiles and necessary for avoidance behavior; the mammalian TRPA homologue has retained its sensitivity to electrophiles throughout evolution (Tracey et al., 2003; Kang et al., 2010). The detection of sound in Drosophila involves ion channels that are related to the TRPV1 family (Kim et al., 2003), but this channel evolved in mammals as one of the key receptors involved in pain signaling. Mammalian TRPA1 and TRPV1 are known to be target molecules of reactive naturally occurring electrophiles (Jordt and Julius, 2002; Xu et al., 2006; Taylor-Clark et al., 2008; Vriens et al., 2008, Riera et al., 2009; Kang et al., 2010; Bohlen et al., 2010; DePetrocellis et al., 2011). However, it is still unclear how these channels become activated by electrophiles. Hinman and colleagues (2006) proposed that the human TRPA1 channel contains three specific cysteine residues, located on the channels’ amino terminus that are targeted by electrophiles and cause channel opening. Macpherson and colleagues (2007) showed similar results in the mouse TRPA1 homologue. Since 2006, numerous groups have shown that TRPA1 cysteines are important for gating and activation by multiple types of agonists. TRPA1 is unique among mammalian TRP channels because it contains a large cysteine-rich amino terminus with 17 ankyrin repeat

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